In certain circumstances, the spinal canal extending through a patient's vertebrae is or becomes too narrow and constricts the spinal cord extending therethrough. Narrowing can be attributable to causes such as age, injury or removal of a spinal disk.
For instance, cervical spondylosis is a common degenerative condition of the cervical spine that most likely is caused by age-related changes in the intervertebral disks. As disk degeneration occurs, mechanical stresses result in osteophytic spurs, which may form along the interior aspect of the spinal canal and can compress the spinal cord. The constriction of the spinal cord in the cervical spine, for example, often produces pain, weakness, or loss of feeling in extremities. Other causes for narrowing of the spinal canal include disc shrinkage, which causes the disc space to narrow and the annulus to bulge and mushroom out, resulting in pressure on the spinal cord. Degenerative arthritis of facet joints can cause joints to enlarge, or the vertebra to slip with respect to each other, also compressing the spinal cord. Instability between vertebrae, such as caused by stretched and thickened ligaments can also produce pressure on the spinal cord and nerve roots.
Myelopathy, or affliction or injury of the spinal cord, occurs due to its compression. The rubbing of the spine against the cord can also contribute to this condition, and the spinal cord compression can ultimately compromise the blood vessels feeding the spinal cord, further aggravating the myelopathy.
Traditional procedures for decompressing the spinal cord include a laminectomy and a laminoplasty. In a laminectomy, the lamina and spinal processes are removed to expose the dura covering the spinal cord. Another known procedure is a laminoplasty, in which the lamina is lifted off the dura, but is not completely removed. According to one laminoplasty procedure sometimes referred to as an “open door” procedure, an osteotomy is performed in which a complete cut is made through one side of the vertebra, approximately between the lamina and lateral mass, while a partial-depth cut is made on the opposite lateral side. The lamina is then hinged open about the partial cut to increase the cross-sectional size of the spinal canal to decompress the spinal cord therein. In certain procedures, a laminoplasty plate is then fixed between the facet and the hinged open lamina. According to some known methods, the plate of an appropriate size is selected and bent to the desired shape and generally has a plurality of screw holes. In other techniques, a strut of bone may be placed in the open portion within the lamina and the facet to help hold the open position of the lamina. In general, prior to the operation, the surgeon needs to measure the vertebra to determine the size of the plate necessary for implantation. At that point, a plate can be selected with the appropriate dimensions, and implanted at the site.